This invention relates generally to gas turbine components, and more particularly to turbine shrouds and related hardware.
It is desirable to operate a gas turbine engine at high temperatures for efficiently generating and extracting energy from these gases. Certain components of a gas turbine engine, for example stationary shrouds segments and their supporting structures, are exposed to the heated stream of combustion gases. The shroud is constructed to withstand primary gas flow temperatures, but its supporting structures are not and must be protected therefrom. To do so, a positive pressure difference is maintained between the secondary flowpath and the primary flowpath. This is expressed as a back flow margin or “BFM”. A positive BFM ensures that any leakage flow will move from the non-flowpath area to the flowpath and not in the other direction.
In prior art turbine designs, various arcuate features such as the above-mentioned shrouds and supporting members are designed to have matching circumferential curvatures at their interfaces under cold (i.e. room temperature) assembly conditions. During hot engine operating conditions, the shrouds and hangers heat up and expand according to their own temperature responses. Because the shroud temperature is much hotter than the supporting structure temperature, the curvature of the shroud segment will expand more and differently from the supporting structure at the interface under steady state, hot temperature operation conditions. In addition, there is more thermal gradient within the shroud than in the supporting structure, resulting in more deflection or cording of the shroud.
Because of these curvature differences between the shroud segment and the supporting structure at the interface, a leakage gap is formed between the shroud segment and the supporting structure and can cause excessive leakage of cooling air, ultimately increasing the risk of localized ingestion of hot flow path gases. These curvature differences also create stresses on the shroud and hanger at the hot temperature condition, lowering the cyclic life of the shroud and hanger. This has led to the use of shroud assemblies which utilize retainers known as “C-clips” to secure the shroud segments to the supporting structure. While the C-clips allow for distortion, they are highly stressed components which present their own problems and can cause serious engine damage if they fail.
Accordingly, there is a need for a shroud design that can reduce the curvature deviation between the a shroud and its supporting structure at hot operating conditions in order to reduce both leakage and stresses at all operating conditions.